The present invention relates to a composition and method for inhibiting limescale formation on and chloride-induced corrosion of a ferrous metal or alloy which contacts an aqueous solution, even solutions of high hardness and high chloride content. As used in the present specification, the term "aqueous solution" refers to an ion-containing solution which is primarily composed of water, such as tapwater or saltwater or the like. As used in the present specification, the term "aqueous solution" includes solids (i.e., frozen solutions such as ice), liquids, or gases (i.e., heated solutions such as steam), or mixtures thereof.
Significant problems exist for systems in which aqueous solutions become concentrated, such as flash heating systems in which feedwater is vaporized by contact with a heat transfer surface. Examples of such flash heating systems include the food storage cabinets used by the quick service restaurant industry, in which water is flash evaporated by contact with a metal heat transfer surface to provide a hot, humid environment for temporary food storage. Problems associated with these systems include chloride-induced corrosion and environmentally-induced cracking, such as corrosion fatigue and stress-corrosion cracking, in systems constructed of ferrous metals and alloys, such as austenitic stainless steels and carbon steels.
Austenitic stainless steels and the like are generally resistant to the corrosive action of most environments due to the formation of a passive oxide film on the surface of the metal. Halides in general and chlorides in particular, however, readily promote localized pitting corrosion of austenitic steels by penetrating this passive oxide film at discrete sites, causing these sites to become electrochemically active compared to the surrounding passive metal. The resulting electrolytic cell causes a corrosion reaction to occur, resulting in localized loss of metal at the anodic (active) site. As the corrosion advances, the continued loss of metal produces a pit in the surface. Once a pit is formed, the local chemical environment within the pit becomes significantly more aggressive than that of the bulk aqueous solution, further driving the corrosion reaction at that site and thus increasing the depth of the pit. While the relative loss of mass caused by pitting corrosion is quite low compared to that caused by generalized corrosion, pitting attack can rapidly cause potentially hazardous failure of metal parts, as the formation of pits severely degrades the structural integrity of the metal body. Indeed, localized corrosion, such as chloride-induced pitting, is responsible for an estimated 90% of metal damage caused by corrosion in the chemical-processing industries.
Pits and other surface irregularities may also serve as initiation sites for other damaging corrosive processes, particularly those involving cracking of the metal. Stress-corrosion cracking ("SCC") is a type of failure in which static tensile stresses, which may be residual or applied, combine synergistically with a chemically aggressive environment to initiate and propagate the formation of cracks within the metal. As the stresses required for SCC are usually much lower than the yield stress of the metal, SCC can cause failure of metal parts held under seemingly acceptable stress levels. A similar cracking process known as corrosion fatigue results from a combination of corrosive environment and cyclic applied stress, such as that produced by the repeated heating and cooling of a metal surface. As with SCC, stress and environment act synergistically to cause cracking of the metal which would not occur in the absence of either factor. Corrosion fatigue can greatly shorten the effective service life of cyclically-loaded metal parts exposed to an aggressive environment.
Pitting and the concomitant environmentally-induced cracking of ferrous metals and alloys can occur in systems using water having only nominal chloride concentrations, possibly as low as 10 parts per million. This is a particular problem in flash heating systems, as the instantaneous vaporization of the feedwater on the heat transfer surface tends to concentrate the chloride ions on that surface and thus creates localized areas of high chloride concentration.
An additional factor contributing to the problems of pitting and environmentally-induced cracking is the formation of scale deposits, particularly on heat transfer surfaces. The most common of these deposits, limescale, typically consists of 90-95% calcium carbonate (CaCO.sub.3) and 5-10% of magnesium carbonate (MgCO.sub.3). The solubility of these calcium and magnesium carbonate salts in the feedwater decreases as the temperature of the feedwater increases. Consequently, flash evaporation of even low hardness water can cause significant limescale deposition on heat transfer surfaces.
Limescale deposits contribute to localized corrosion by preventing oxygen from migrating to the metal surface underneath a deposit. This creates an oxygen differential cell in which the surface beneath the deposit is anodic relative to the surrounding metal. The resulting corrosion reaction causes localized metal loss at the anode, as well as a buildup of corrosion products beneath the deposit. Through various mechanisms, these corrosion products act to increase the aggressiveness of the local environment of the anode, further promoting localized corrosion. Scale also acts as an insulator, requiring increased metal temperatures to achieve the same heat transfer rates across a portion of scaled metal. In flash vaporization systems, these higher temperatures increase the magnitude of the stress cycle created by the constant heating and cooling of the heat transfer surface, thus further promoting corrosion fatigue of that surface.
In order to overcome these problems, one approach has been pretreatment or conditioning of the feedwater in an effort to reduce the concentrations of deleterious ions. Several methods have been developed to accomplish this reduction, including the use of reverse-osmosis and ion-exchange filtration units. These methods, however, while providing significant reductions in the ion content of the feedwater, cannot completely remove all of the deleterious ions from the feedwater. Consequently, these methods slow development of corrosion and scale but cannot completely eliminate their formation. Additionally, the use of high purity feedwater may not be economically feasible, or even practical, for some processes.
An alternative approach to reducing corrosion involves the addition of corrosion inhibitors to the feedwater. These corrosion inhibitors are generally classified according to which half of the corrosion reaction they inhibit.
More specifically, anodic inhibitors prevent the migration of metal ions from the metal surface into the water, thus inhibiting the anodic half of the reaction. Anodic inhibitors typically act by initiating or promoting the formation of an oxide film on the metal surface. These inhibitors, however, are usually sensitive to the pH and chloride ion concentration of the feedwater, and generally have a critical minimum concentration, below which the inhibitor will actually facilitate localized corrosion. Because of this problem, frequent monitoring of inhibitor level is generally required when using anodic inhibitors. Additionally, many anodic inhibitors are undesirable because of toxicity and other environmental concerns. Examples of typical anodic inhibitors include chromates, nitrites, molybdates and phosphates. These inhibitors may be used individually or in combination with one another or even together with cathodic inhibitors.
Unlike anodic inhibitors, cathodic inhibitors affect the cathodic half of the corrosion reaction, which is typically the reduction of oxygen to form hydroxyl ions. Cathodic inhibitors typically act by producing complexes that migrate to areas of locally elevated pH, forming barriers that isolate the cathode from the aqueous solution. Cathodic inhibitors are generally less effective corrosion inhibitors than anodic inhibitors. Additionally, some cathodic inhibitors exhibit poor hydrolytic stability and may thus degrade to scale-forming ions under certain conditions. The performance of cathodic inhibitors may also be adversely affected by solution temperature and pH. Examples of typical cathodic inhibitors include zinc ions, polyphosphates and phosphonates. Cathodic inhibitors may be used individually or in combination with other cathodic and/or anodic inhibitors.
Limescale formation on heat transfer surfaces may also be controlled by chemical treatment of the feedwater. In the past, such treatment usually involved addition of strongly acidic compounds, such as sulfuric acid, to the feedwater, causing a shift in the solubility equilibrium away from formation of calcium carbonate.
The more common treatments in use today tend to focus on inhibiting either the nucleation or crystal growth stages of the scale formation process. For example, polymeric inorganic phosphates and phosphonates adsorb onto nuclei and growing limescale crystals, thus inhibiting both nucleation and crystal growth. Under certain conditions, however, the polyphosphates can hydrolyze to form phosphate ions, which decreases the inhibitory action of the additive as well as promoting the formation of insoluble calcium phosphate scale. Similarly, some phosphonates may be degraded to phosphate ions in aqueous solutions containing even low concentrations of chloride ions.
Organic polymers such as carboxylic acid polymers are also presently used to control scale formation. These polymers, for example polyacrylates, polymaleates and polymethacrylates, typically act as dispersants and prevent scale deposition by keeping small particles of scale in suspension. Many carboxylic acid polymers, however, exhibit dramatically reduced efficacy in water of high hardness, limiting their usefulness in systems where aqueous solutions become concentrated.